The present invention relates to shaping a fiber preform by winding a fiber texture obtained by three-dimensional (3D) or multilayer weaving so as to constitute a fiber reinforcing structure for a composite material part.
More particularly, the invention relates to fiber preforms that are to form reinforcement in composite material parts that are bodies revolution of profile that varies in radial section (varying in shape and/or in thickness), such as for example an aeroengine fan casing.
The fiber preform in such a part is made by weaving a fiber texture and by winding the texture under tension onto a mold in the form of a body of revolution having a winding surface of radial section presenting a profile in relief corresponding to the profile of the part that is to be fabricated.
In order to make a fiber texture that matches the varying shape of the injection mold, use is made of shaped weaving, also known as “contour” weaving or “outline” weaving, which consists in taking off lengths of warp yarns that differ as a function of yarn position across the width of a fiber texture that is being being woven in the form of a strip so as to obtain ratios between the lengths of warp yarns (in the tangential direction) that are similar to the ratios between the radii defining the varying profile of the final part that is to be fabricated.
The fiber texture as woven in this way is stored on a storage mandrel for subsequently winding under tension onto an injection mold in the form of a body of revolution having a winding surface having a profile in relief in radial section that corresponds to the profile of the part that is to be fabricated. While it is being wound onto the mold, the texture is tensioned by the storage mandrel, also referred to as the “take-up” mandrel, with one or more follower rollers being placed between the storage mandrel and the injection mold. In the axial width direction, the or each follower roller presents a radius that varies, defining an outer surface with a profile in relief that serves to conserve the yarn length ratios as defined during the weaving between the storage mandrel and the injection mandrel.
Nevertheless, while the fiber preform is being formed on the mold, the fiber texture is wound onto itself in a plurality of turns. On each winding turn, the ratio between the various radii defining the profile in relief changes as a function of the number of fiber texture turns that have already been wound. The magnitudes of these changes in profile increase with increasing shape and/or thickness ratios in the final part. However, the above-described shaping method allows the fiber texture to be driven under tension with only one profile in relief, i.e. with radii presenting a cross-section ratio that is constant.
While being wound onto the injection mold, the fiber texture comes out of register because of the differences between the profile as woven and the real profile onto which the texture is being wound, thereby giving rise to losses of tension in certain axial positions in the texture. Such unbalances in tension across the width of the part can give rise to numerous defects such as waves, fibers buckling, pinched fibers, zones of unwanted extra thickness, and fiber volume contents that are out of specification. Such unbalances in tension also make the shaping of the fiber texture by winding more complicated, in particular by giving rise to the formation of creases or misalignments, thereby making the shaping of the fiber texture more arduous and time-consuming.